This invention relates to a mould for use in the continuous casting of billets of metals with a high melting point, particularly iron and steel, and to a method of manufacturing such a mould.
Copper is widely used in continuous casting moulds, because of its good thermal conductivity and because it is fairly strong. Frequently, copper is used in a continuous casting mould as a substrate for other metals, which are typically selected for their strength, lubricity, and hardness.
U.S. Pat. No. 4,037,646 discloses a mould having a substrate made of copper, an innermost layer of nickel and/or cobalt which, in addition, contains phosphorus or boron, and a layer of nickel and/or cobalt which binds the innermost layer to the copper substrate. On exposure to heat during the casting process, the innermost layer of the mould described in Pat. No. 4,037,646 increases in hardness, e.g. from about 400 HV to 1000 HV.
German Patent Application No. 3,336,373 describes a mould having an inner layer that is formed by electrolytic precipitation and is composed mainly of nickel. The strength of the inner layer may be increased by incorporating particles of silicon carbide in the nickel.
European Patent Application No. 0,125,509 discloses a mould having a substrate made of copper on which an inner layer of a hard wearing material, such as nickel, is deposited. The thickness of the inner layer is about 35% of the total thickness of the mould (substrate plus inner layer). The inner layer may be fiber-reinforced in the manner described in German Patent Application No. 3,038,289.
In the case of European Patent Application No. 0,125,509, the copper substrate is manufactured by electroforming, i.e., electrolytic precipitation of copper onto a form. Electroforming is conventionally used for manufacture of waveguides and masters for phonograph records.
Japanese Patent Application No. 56-154,262 discloses a mould having the construction shown in FIG. 1. This mould comprises a substrate 2 of copper or copper alloy, a reinforcing layer 3 of nickel or nickel/cobalt alloy containing fibers of SiC, Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4 and particles of wear resisting material, such as WC, Si, alumina, TiN or Cr oxide. The mould also has an innermost layer 4 of chromium.
Chromium is generally used in a continuous casting mould because of its strength and lubricity. However, when chromium, which is brittle, is deposited over copper, which is soft, it is liable to fracture. The reinforcing layer of nickel or nickel/cobalt alloy is employed in Japanese Patent Application No. 56-154,262 because it is harder than copper and reduces the likelihood that the layer of chromium will break.
Each of the methods described above for manufacturing a mould has the disadvantage that the material of the substrate, i.e., copper, is not very strong, and the strength of the mould is not increased substantially by plating. Consequently, the mould is easily deformed in use and tends to bulge, which may take place irrespective of the cooling of the substrate. A plating containing reinforcing material, as described with reference to Japanese Patent Application No. 56-154,262, for example, does not increase the strength of the mould substantially.